1. Field of the Invention
The present invention relates to an ink composition favorably used for inkjet recording, an inkjet recording method, a printed material produced therewith, a planographic printing plate produced using the ink composition, and a method for producing a planographic printing plate.
2. Description of the Related Art
Cyclic ether compounds such as 3-membered rings and 4-membered rings are known to exhibit high reactivity, and are used as polymerizable compounds contained in curable compositions subjected to photocationic polymerization or heat polymerization using an acid anhydride (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 11-43540 and 11-60702.)
There are many image-recording methods of forming an image on a recording medium such as paper based on image data signals, including those in electrophotographic process, sublimation or fusion heat-transfer process, and inkjet process. Among them, the inkjet process is advantageous in that it allow printing in a cheaper device at a lower running cost, because it forms an image directly on a recording medium by ejecting ink only in desirable image region and thus uses the ink more efficiently. In addition, the inkjet process is also less noisy and thus advantageous as an image-recording method.
The inkjet process allows printing not only on plain paper but also on non-water-absorptive recording media such as plastic sheet and metal plate, but recently, there is an urgent need for acceleration of printing and improvement in image quality in the process. The period needed for drying and curing the ink droplet after ejection has a great influence on the printing efficiency and the quality of printed image.
In the inkjet recording processes, there is a recording method by using an inkjet recording ink that cures by irradiation with radiation ray. In the method, it is possible to improve the printing efficiency and the quality of image, by curing the ink droplet by irradiating it with radiation ray immediately or after a particular period from ejection.
If it is possible to raise the sensitivity of such an inkjet recording ink that cures by irradiation with radiation ray such as ultraviolet light or to raise the efficiency of the ink curing by radiation ray, it is possible to obtain many benefits, such as improvement in inkjet recording efficiency, reduction of power consumption, elongation of the lifetime of radiation ray generator due to decrease in the load, and prevention of evaporation of low-molecular mass substances due to insufficient curing. In addition, the improvement in sensitivity is effective in increasing the strength of the image formed with inkjet recording ink, and in particular, when the ink composition is applied to preparation of planographic printing plates, it leads to increase in the hardness of the image part and thus to increase in printing durability.
In recent years, curable inkjet systems employing a radiation ray such as ultraviolet light have been receiving attention because they give relatively less odor, produce fast-drying images, and allow recording on a recording medium which does not absorb ink. A cationic polymerizable ink composition which provides excellent adhesiveness to a recording medium with low shrinkage when exposed to ultraviolet light has been proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 9-183928). However, the cationic polymerizable ink is not sufficiently stable during storage because of a reaction caused by acid generated over time, which is an obstacle to the commercialization of the ink. Therefore, in order to improve the storage stability, a technique of adding a basic compound or a thermal base generator has been proposed (see, for example, JP-A Nos. 2003-312121, 2003-341217, and 2004-91558). However, there is another problem that the curing sensitivity of the ink deteriorates because the basic compound inhibits the function of the acid generated upon exposure.
However, in recent years, a technique of digitizing image information using a computer by electronically processing, storing, and outputting the information has become widespread, and a new image output system that can be used for the above technique has been demanded. In particular, a method for making a printing plate without treatment with a developing solution, and a method for directly making a planographic printing plate using an inkjet recording ink composition, have been investigated (see, for example, JP-A No. 54-117203). These methods contain steps of image-wisely ejecting ink onto a support surface, which is preferably hydrophilic, and curing the ink by irradiation with an active radiation ray to make a printing plate having a desired image (preferably a hydrophobic image). For the formation of image areas on the planographic printing plate, it is desired that, for example, ink droplets ejected on the support immediately cure with no bleeding, the cured image areas have excellent strength and adhesiveness to the support, and the image areas sufficiently conform to the bending of the support and receive no damage such as cracks when the planographic printing plate is mounted at a printing machine. Accordingly, an ink composition suitable to these applications is needed. In particular, a multifunctional cyclic ether, which is used for the purpose of improving the sensitivity of cationic polymerizable ink and cures to form a polymer having a highly crosslinked three-dimensional structure, needs to be improved to solve the problem of the deterioration of the flexibility of the cured ink coating.